1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus including the optical scanning device, such as a digital copier, a printer, a plotter, and a facsimile machine.
2. Description of the Related Art
Conventionally, there is known an optical scanning device for deflecting a light beam with a deflecting unit such as an optical deflector, imaging the deflected light beam as microscopic spot light onto a surface which is a target of the scanning operation (scan target surface), and scanning the scan target surface in a main scanning direction at a constant speed. This technology is applied as a latent image writing unit in an image forming apparatus such as a laser beam printer, a laser beam plotter, a facsimile machine, and a digital copier. This optical scanning device scans a scan target surface such as an image carrier by using an optical deflector to deflect/reflect a laser beam radiated from a laser light source. At the same time, the intensity of the laser beam is modulated in accordance with image signals (e.g., ON, OFF). Accordingly, an image is written onto the scan target surface.
A rotatable polygon mirror that rotates at a constant speed is widely used as an optical deflector. An fθ lens is generally used as the scanning lens for imaging the deflected light beam onto the scan target surface. With the use of the fθ lens, the scanning speed of the deflected light beam can be made substantially constant at the respective scanning positions on the scan target surface, and the beam spot diameters can be made substantially consistent.
However, with an optical scanning device using the polygon mirror and the fθ lens, there are cases where the scanning speed at the respective scanning positions on the scan target surface cannot be corrected so as to be constant over the entire effective writing region. This correction failure may occur when a thin scanning lens is used for the purpose of reducing cost. Moreover, the static beam spot diameters are generally substantially consistent at the respective scanning positions in an optical scanning device using the polygon mirror and the fθ lens. Thus, if the light source were modulated by the same light emitting pulse widths at the respective scanning positions, the respective scanning speeds would vary, and therefore the scanning lengths would be inconsistent. As a result, the scanning beam spot diameters would be inconsistent. If the photoconductor surface were exposed to scanning beam spots having different diameters at the respective scanning positions, the dot diameters would be inconsistent and the image would include regions having inconsistent densities. This leads to degraded image quality.
Meanwhile, there have been proposals of using a micro mirror as the optical deflector. A micro mirror has a resonance structure using the micromachining technique, and performs sine wave oscillation. When a rotatable polygon mirror is used, the apparatus needs to be increased in size and high-speed mechanical rotation is performed. Accordingly, problems arise such as banding caused by oscillation, increased temperature, noise, and increased power consumption. However, when a micro mirror is used as the deflecting unit of the optical scanning device, the apparatus can be reduced in size, and the above problems, such as banding caused by oscillation, increased temperature, noise, and increased power consumption, can be significantly mitigated. That is, by using a micro mirror that performs sine wave oscillation instead of a polygon mirror, noise and power consumption can be reduced, thus providing an image forming apparatus that is suited for the office environment. Such an image forming apparatus is also environmentally friendly because power consumption can be reduced.
However, if a micro mirror that performs sine wave oscillation were used as the deflecting unit, the deflection angle would change according to sine waves. Thus, when an fq lens used in current writing optical systems is applied to the scan-imaging optical system, the scanning speed decreases at the peripheral image heights. Accordingly, the scanning speed would be non-constant on the scan target surface. In this case also, the image quality would be degraded as described above.
To address this problem, patent document 1 discloses a technique of using a scan-imaging optical system (f·arcsin lens) having imaging properties (f·arcsin properties) as indicated by the following formula.H=K×sin−1(f/2f0)                (where H: image height, K: proportional constant, f: deflection angle, f0: amplitude)Accordingly, such a technique provides an optical scanning device capable of optically correcting the waist position of a main scanning light beam, whereby the optical scanning device has a large effective writing width and performs the scanning at a constant scanning speed. However, by performing the above-described optical correction, there will be increased deviation among spot diameters of main scanning light beams at respective image heights on the scan target surface. This leads to degraded image quality.        
As described above, in an optical scanning device using a micro mirror as the deflecting unit for performing sine wave oscillation, there is a trade-off relationship between having a constant scanning speed and the deviation among spot diameters of main scanning light beams at respective image heights on the scan target surface. There is yet to be proposed an optical scanning device which has favorable properties in both of these respects and which is capable of forming high-quality images.
Patent document 2 discloses an optical scanning device using a micro mirror as the deflecting unit for performing sine wave oscillation. Specifically, a consistent amount of light is emitted to the respective scanning positions by controlling the light emitting time of the light source, without using a scan-imaging optical system. However, if a scan-imaging optical system were not used, a considerably large curvature of image field would appear on the scan target surface. Accordingly, the diameter of the light beam cannot be reduced, and therefore high-resolution images cannot be formed. Furthermore, variations in optical properties would increase with respect to tolerance variations of optical elements, which would make it difficult to manufacture scanning devices with stable quality.    Patent Document 1: Japanese Laid-Open Patent Application No. 2005-215571    Patent Document 2: Japanese Laid-Open Patent Application No. 2007-292918